<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style>
      .active1 {
        background-image: url(https://gimg2.baidu.com/image_search/src=http%3A%2F%2Fimg.jj20.com%2Fup%2Fallimg%2F1113%2F060120105F7%2F200601105F7-1-1200.jpg&refer=http%3A%2F%2Fimg.jj20.com&app=2002&size=f9999,10000&q=a80&n=0&g=0n&fmt=jpeg?sec=1647595321&t=213e1ba3baca6fa3399a607d249db0bd);
        background-size: 100vw 100vh;
      }

      .active2 {
        background-image: url(https://gimg2.baidu.com/image_search/src=http%3A%2F%2Fup.enterdesk.com%2Fedpic%2F26%2Fc8%2F69%2F26c869bc8428113a454b0207e42f339f.jpg&refer=http%3A%2F%2Fup.enterdesk.com&app=2002&size=f9999,10000&q=a80&n=0&g=0n&fmt=jpeg?sec=1647595321&t=93b44ed578b4fe7721f43b19700017a3);
        background-size: 100vw 100vh;
      }
    </style>
  </head>
  <body>
    <button>按钮一</button><button>按钮二</button>
    <script>
      var btns = document.querySelectorAll("button");
      // btns[0].onclick = function(){
      //     document.body.className = "active1";
      // }
      // btns[1].onclick = function(){
      //     document.body.className = "active2";
      // }

      for (let i = 0; i < btns.length; i++) {
        btns[i].onclick = function () {
          // console.log(i);
          document.body.className = `active${1 + i}`;
        };
      }
    </script>
  </body>
</html>
